There's a lot going on in your question which makes sense considering you're expressing that you don't get where some people are coming from. I will try to address two of the things you raise and hope that this answers what you meant to ask.
Problem 1: Absolute Standards
When you doubt the logic of absolute standards in ethics, I think the trick to all such views lies in a rejection of:
Surely there is less evil in one individual dying that many.
Reworded, this means that you subscribe to a quantitative approach to good and evil. Generally, this means a consequentialist approach to ethics where the value of an action is judged by whether or not it produces the "best" consequences (where best could be any of a variety of things, such as maximizing happiness, maximizing freedom, or minimizing pain).
Approaches to ethics that assert absolute values reject the idea that all ethical goods can be quantified. Instead, they take it as prima facie that some ethical goods cannot be sacrificed regardless of other consequences.
A practical example might help here. On a merely quantitative approach, the answer to the following type of thought experiment is crystal clear:
IF you could go back in time and kill Pol Pot when he was 5 years old before he murdered all those people and made Cambodia suffer, should you do it?
Assuming our one principle is minimizing suffering, then this absolutely seems like the right thing to do. But many of us would question the rightness of killing a five-year old regardless of the probable future of that five year old. (I don't here want to debate the merits of these views -- just mention their takes). On such accounts, there are values that are absolute and should be extended to say all sentient beings such as the right to live until they are violating the rights of others.
A further consideration in this problem is that consequentialist and utilitarian views are not immune to it. Instead, they seem committed to asserting some quantity is qualitatively valuable (Why should we care about suffering it lacks objective importance?)
Problem 2: Conflicting Principles
A second issue you raise is the question of conflicting principles. This is a common objection to Kant's ethics. Sometimes the objection is raised is confused terms that misunderstand what Kant is doing, so we'll skip over Kant and the specific issues his moral philosophy raises.
If we instead look at Aristotle, there are multiple qualitative features that are important for being ethical. Some times they do conflict but then we need to balance them based on what adheres to a life of excellence -- something which Aristotle claims someone with moral wisdom can do.
A second approach, which is Kant's, is to reject the claim that these things can come into conflict. Kant's trick is complex but we can boil it down (for the purposes of responding to your objection) to qualitatively valuing reason in free rational creatures above all else and to understanding ethics as the application of reason to questions of action.
a further consideration is that again consequentialist views do not lack conflicts in principles. Mill's Utilitarianism asserts that actions are good to the extent they promote happiness and bad to the extent they promote unhappiness, but he also incorporates at least two other principles that seem in conflict with this. First, in the same text, he tries to claim there's a distinction in types of pleasure based on what should make us happy. Second, in On Liberty (and elsewhere), he asserts that we cannot harm anyone to maximize happiness -- but this means there's two principles in his consequentialist view: the greatest happiness principle and the harm principle.
In other words, conflicting principles seems to be a nearly endemic problem... but generally a problem asserted by the critics of any particular view which the defenders (leaving aside Kant's approach) think indicates multiple but non-contradictory values.
Note well, I'm not saying you need to accept these views. Instead, I would say:
- Views that think there are inviolable principles usually do so on qualitative grounds rather than quantitative ones.
- The conflict of principles problem is a common objection but not identical to a proof that there aren't multiple things we value.
To clarify based on the OPs addition to their question, philosophical accounts that have absolute principles use these to create absolute prohibitions on certain actions.
For instance, if you believe it is always wrong to use violence presumably you believe so because you think doing violence is qualitatively an absolute wrong that cannot be cancelled out by some other benefit (i.e. you cannot match quality with any quantity).
To repeat, the justification is following Kant precisely that you take something to be of absolute worth and set this in contrast with things that have a price. Things of absolute worth are not subject to calculation.
For instance if you take intelligent life to be of absolute value (qualitative claim), include dolphins in the definition of absolute value (empirical claim), then you conclude there's an absolute prohibition on killing dolphins.
If you object, "but what if ..." and pose some hefty moral cost for this (every child on earth loses an eye, a kidney, and a leg), then one of two things will occur:
- The person who claimed to place an absolute value will fold on that position and convert to a quantitative claim about the goodness of saving dolphins
- They will hold onto the qualitative claim and say that the wrong that would happen would happen.
There's actually another step usually involved. Generally, the person who believes everything is quantitative makes an assumption not shared by the people who hold to qualitatively distinct absolute values, and that assumption is about agency.
In the example I provided, it's really hard to imagine why or how not killing dolphins would result in this consequence of every child losing 3 things. Generally, the next step is for the objector to move to a more plausible example, but there's several issues:
First, you'll never get a completely convincing causal story that places the qualitative good directly opposite a quantity of some good. This is because many of the things people qualitatively value (such as freedom) relate to agency, and make it so that it's rarely if ever convincing that protecting the qualitative good is the cause of the quantitative loss. So then why should it be abandoned or reduced?